Electrodeposition of chromium and duplex micro-crack chromium coatings



United States Patent Otlfice 3,418,220 Patented Dec. 24, 1968 3,418,220ELECTRODEPOSITION F CHROMIUM AND DU- PLEX MICRO-CRACK CHROMIUM COATINGSWilhelm Roggendorf, 3 Alte Zollstrasse,

7460 Kehl am Rhine, Germany No Drawing. Filed Aug. 18, 1964, Ser. No.390,463 Claims priority, application Germany, Aug. 26, 1963, R 35,983 17Claims. (Cl. 204-41) ABSTRACT OF THE DISCLOSURE A chromium pl-atingaqueous solution is used in which tetrachromate of the type of sodiumtetrachrornate (Na Cr O or which may also be expressed as Na O-4CrO isthe principal, predominant, basis or major constituent and in which alesser but appreciable amount of free chromic acid is also aconstituent, This primary part of the plating solution may be obtainedby adding chromic acid to water and then adding sodium hydroxide orcarbonate to convert a greater part of the chromic acid to sodiumtetrachromate. The solution also employs a chemical complex of dissolvedmaterials in relatively minor amounts. Such materials include a sulfatewhich may be provided by adding sulfuric acid; cations of metals of theclass of titanium and zirconium (essential) and fluoride which may beprovided by the addition of a sodium fluoride of one or both of themetals; soluble silicon; and selenium. The silicon may be provided byadding silicon fluoride and selenium by adding selenium dioxide. Wheresilicon is to be used, it may be added as silicon fluoride and thetitanium and zirconium may be added in the form of potassium titanateand zirconate. The solution is maintained within a, relatively lowoperating temperature range of about 18 to 39 C., preferably about 18 to32 C. when the solution is used without selenium and 30 to 32 C. when itis used with selenium. The selenium is particularly useful when a dualcoating is to be provided on a metal workpiece. The solution has a ratioof total chromic acid to free chromic acid content of about 1 to 3 up toabout 1 to 5.

This invention relates to the provision of an improved chromium platedsurface on metal articles or objects and particularly, to. a process forproviding a corrosion-resistant, crack and pore-free chromium plating ona metal surface. A phase of the invention relates to providing amicro-cracked, chromium plated surface of improved characteristics andparticularly, to a double chromium layer type of metal surface coatingor deposit that has a crackfree first or initial layer or deposit and asecond microcracked layer or deposit.

An object of my invention has been to provide an improved brightchromium plated metal surface or coating which will essentially have animproved resistance to corrosion.

Another object of the invention has been to provide a stress-freechromium coating on a metal surface that is porefree and bright.

A further object of the invention has been to provide an improvedchromium plating process and one which is more efficient and effectivefrom the standpoint of the action of its electrolytic solution or bathin depositing chromium on a metal surface.

A still further object has been to provide a dual coating or deposit ofchromium whose first coating will be crackfree and whose second or finalcoating will have a microcracked surface pattern, such that the overallcoating will have an improved resistance to corrosion.

These and other objects of my invention will appear to those skilled inthe art from the following description and the appended claims.

Heretofore, chromic acid basis electrolytes with catalysts have beenused for electroplating corrosion-resistant chromium coatings or layers,but such solutions for effective results have been used at temperaturesabove 40 C.

It has also been known that for the deposition of crackfree chromium,using a sulphate catalyzed acid-chromium plating solution, that thenumber of cracks per square centimeter of surface area will depend uponthe ratios of the amount of chromic acid to the catalyst, and on thetemperature of the electrolyte used. As an example, the number of cracksper square centimeter of surface area will be reduced in an electrolyteoperating at a ratio of chromic acid to sulfuric acid of 100 to 1, ifthe operating temperature is raised from the norm-a1 range of to highertemperatures of or C. Also, a reduction of cracks per square centimeterof surface area will occur if the temperature is held at the normalrange of 45 C., if the ratio of chromic acid to sulfuric acid catalystis raised from to 1 to -150 to 1.

It is also known that more or less crack-free chromium layers may bedeposited from a chromic acid solution containing as the catalyst,sulfuric acid or mixed catalysts of sulfuric acid and silicofluoric acidto which electrolyte magnesium oxide is added and temperature of thesolution is raised above about 40 C.

The detrimental or disadvantageous results of the abovementionedso-called crack-free solutions is that, in reducing the number of cracksper square centimeter of surface area, the porosity will be increased.Thus, the advantage of a reduced number of cracks per unit of surfacearea is offset by the increased porosity and the resultant loss ofcorrosion resistance incident thereto.

It is also known that when the above-mentioned socalled crack-freechromic acid plating solutions are used, the deposit is plated underhigh internal stress. Thus, when the chromium plated objects or articlesare mechanically deformed, as for example, during assembly with pressureor pull, when subjected to large variations in temperature, or whensubjected to deformation by bending in use or by abrasive action ofstones hitting the plated objects, the original so-called crack-freesurface is destroyed. In addition, in view of the inherent internalstress of the chromium coating or plating, it will show a coarselydistributed crack pattern per unit of surface area.

Another factor is that chromic acid plating solutions containingsilicofluoride catalysts or mixed catalysts, such as sulphate andsilicofluoride catalysts, will tend to age. The use of an agedelectrolyte produces a bright chromium plating with a coarsely crackedpattern.

Chromic acid plating solutions heretofore used for micro-porous chromiumdepositing have only a narrow plating range insofar as cathode currentdensities are concerned in which the micro-porous deposit will appear,and under and over which range micro-porosity that is desired is notaccomplished. In view of the well-known bad throwing power of chromicacid plating solutions, the ability to maintain micro-porous deposits onprominent and, at the same time, on deeply recessed areas on the samearticle or object becomes very difficult if not impossible to achieve.

In accordance with my invention, I have discovered that it is possibleto achieve an electro-deposit of chromium that is crack and pore-free aswell as bright or that is additionally micro-cracked and isexceptionally useful for double chromium plating. This is accomplishedby using a sodium tetrachromate basis electrolyte that containsbrighteners or complexes of soluble titanium and Zirconium compounds,with the preferred or optimum complex being a fluoride of the namedmetals and an additional metal selenium.

I have determined that the addition of 1 /2 grams of titanium fluoride(TiF or 4 grams of zirconium fluoride (ZrF per liter of electrolyte willprovide a tetrachromate electrolyte with the same effects of reductionin cracking tendencies as occurs in a sulphate-catalyzed chromiumplating solution having a chromic acid to sulphate ratio of 100 to 1when the temperature of the solution is raised from 40 to 60 C. However,the bright chromium deposit obtained from the use of my tetrachromateelectrolyte will provide a very low value of internal stress or, inother words, a substantially stress and strain-free structure, ascompared to a deposit gained from the abovementioned known platingsolutions. Further, additions in the magnitude of 3 grams of titaniumfluoride or 8 grams of zirconium fluoride in the electrolyte will leadto an even greater reduction in tendency for crack formation or, inother words, to an optimum stress-free structure.

A bright chromium plating solution of a tetrachromate basis containingcomplexes of titanium and zirconium, with and without selenium, operatesat an electrolyte temperature below 32 C. and, as an optimum, in therange of about 29 to 31 C. It will provide bright electro deposits in awide current density range of about 10 to 20 amperes per squaredeeimeter, and will produce a brillant and micro-cracked appearance anda coating having an extremely good corrosion resistance. 1 havediscovered, in view of the unsurpassed high throwing power of anelectrolyte of a tetrachromate type, that the chromium deposit Will beaccomplished on objects having high prominence and deep recesses withoutthe necessity of resorting to special techniques, such as high currentstrikes or internal anodes, etc.

In accordance with my invention, a bright plating solution of atetrachromate type is most suitable for double chromium plating ofbright nickel plated objects or of objects that were earlier doublenickel plated. Such objects may be chromium plated by using the solutionof Example I, in which they may be chromium plated to a thickness ofabout 0.25 micron to produce a crack-free deposit or coating. The workpieces or objects may then be re moved in a manner avoiding anodicpolarity by cathodic charging during the transfer, at a potential belowthe deposition potential of chromium, for example at about 1 to 2 volts.That is, the objects to be provided with a second coating are removedfrom the solution of Example I and may then immediately, withoutrinsing, be immersed in the solution of Example II.

In the second solution, the Work pieces, objects or articles may besubjected to a step-wise increased current density and plated at a finalcurrent density within a range of about 10 to 20 amperes per squaredeeimeter at a solution temperature of about 29 to 31 C. to anelectroplated thickness of, for example, about 0.5 micron. This producesa final, micro-cracked, bright chromium coating or deposit. The drag-inof electrolyte from solution I into the electrolyte of solution II isnot harmful, but has been found to be helpful, since both electrolytecompositions or solutions are compatible.

German patent DRP 608,757 to Bornhauser indicates that his tetrachromatebasis or type of solution will yield a chromium deposit which is matteand never bright, requiring subsequent finishing; it is not crack andpore free or micro cracked. Such solutions are characterized asoperating at higher than usual current densities for chromium plating(200 to 900 amp/sq. ft.), and at low temperatures Within a range ofabout 16 to 22 C. and that should not exceed 24 C. According toBornhauser, sodium tetrachromate will decompose spontaneously if heatedabove 40 C. into complexesof lower molecular sizes. It is indicated thatat higher temperatures such a solution loses its characteristicadvantages.

As contrasted to the above, I have found that electroplating solutionsof my formulations of sodium tetrachromate type or basis are extremelystable under high temperatures and will not decompose when thetemperature is raised to 30 C. or even higher, about up to 60 C. Infact, I have determined that a good upper limit of their working rangeis about 32 C. Further, my electrolytes do not lead to a sudden drop ofcathode current efliciency. I have found that plating at 2 amperes persquare centimeter with an electrolyte temperature of 30, the cathodecurrent efliciency was 24% of the theoretical. This efliciency wasreduced at a current density of 20 amperes per square deeimeter and anoperating temperature of 35 C. by only 2%, with an average currentefliciency of 22% of the theoretical that I have found when operating inthis range. This is considerably less reduction in current efficiencythan previously reported as characteristic of a tetrachromate basischromium plating solution and was not and could not have been predicted.

In all the examples and the discussion of my invention, I have referenceto grams per liter of solution when grams are mentioned, to a solutionof a sodium tetrachromate type, and to an aqueous solution that is madeup from water to the required chromic acid concentration.

Example I A compatible metal work piece or object, such as a brightnickel plated object, was chromium plated in a solution of mytetrachromate basis of the following composition per liter ofelectrolyte:

In this example, a solution temperature of 25 C. with a cathodic currentdensity of 12 amperes per square decimeter was employed for 14 minutes.It provided a chromium deposit free of cracks and pores, with anasdeposited, commercially acceptable brightness.

Example II A workpiece having a compatible surface, such as one nickelplated with a double nickel layer, was chromium plated, using an aqueouselectrolyte solution of a tetrachromate type containing the followingcomposition per liter of solution:

Grams Chromic acid anhydride 280-320 Free chromic acid -120 Sulfuricacid 1.7-1.9 Titanium fluoride 0.1-0.5 Zirconium fluoride 0.3-1.2

Selenium dioxide 0.01-0.02

The above solution thus contains 1.66 to 1.86 grams of sulfate ions, .03to .15 gram of titanium ions or .13 to .53 gram of zirconium ions and.07 to .67 gram of fluoride ions, as calculated from the amounts of thesulfuric acid and titanium or zirconium fluoride present.

The solution was maintained at a temperature of about 30 C. and theplating was effected at a cathode current density of 16 amperes persquare deeimeter for ten minutes. The resultant chromium coating ordeposit showed a micro-cracked appearance with more than 1,000 cracksper square centimeter of surface areas. A double chromium platingaccomplished by using the solutions of Examples I and II in successionhas the additional advantage that the throwing power is maximal inobtaining the first crack-free chromium deposit at :a solutiontemperature of about 25 C. (using solution I). Thus, the coveringability is greatly enhanced for a subsequently produced microcrackedchromium deposit (using solution 11).

In carrying out my invention, I make use of a tetrachromateelectroplating solution containing a major amount or quantity of chromicacid converted to sodium tetrachromate (Na Cr O as the predominant orbasic chromium containing constituent, and a lesser but appreciableintermediate amount of free chromic acid I have been able to provide agreatly improved chromium coating or deposit on a metal surface of aworkpiece and to increase the effective throwing power of the solutionby employing a chemical complex represented by a catalyst containing aminor amount of a sulphate and a fluoride, and brightness represented bya minor but effective amount or quantity of a soluble metal compound ofthe class consisting of titanium and zirconium, with and Without siliconor selenium, also used in the form of a soluble compound. That is, I usea minor amount of a chemical catalyst complex, as, for example, providedby sulfuric acid or sodium sulphate for the sulphate, and as provided byalkali metal fluorides of the class of metals consisting of titanium andzirconium or by fluorides of the class of elements consisting of sodiumand silicon for the fluoride, to not only provide a more effectivethrowing power and a lesser current density, but also to employ a widerrange of temperature and particularly, to effectively employ a lowerrange of temperature of the plating solution. Further, the chromiumcoating as applied is pore-free, highly resistant to corrosion and tostrain and stress produced cracking, is crack-free for any desiredinitial deposited coating thickness, and is bright as directly depositedon the metal surface of the work article or piece.

In such a solution, it appears that the fluoride and the sulphate workas catalysts, that the metals of the class of zirconium and titanium,with or without soluble silicon or selenium additions, work in providinga bright surface and in producing a pore-free, stress-free, crack andcorrosion resistant coating. The increased throwing power appears toresult from the complex of chemicals and particularly, from the complexof fluoride and sulphate catalysts with the metal cations. It will benoted that the titanium and zirconium cations may be provided separatelyor as a mixture in the solution, but should not exceed specifiedquantities for each, whether present individually or as a mixture.

1 have further discovered that selenium may be used in a relativelyminute quantity in such a plating solution to augment the action of themetals of the class of zirconium and titanium. Further, I havediscovered that selenium may be used in a slightly larger amount in asolution used for applying a coating or coatings to produce a singlemicro cracked deposit or dual coated chromium surface whose outer orlast coating is desirably provided with highly minute or micro cracks.

By using a combination or complex of fluoride and sulphates with thebrightening metals in soluble form, I have been able to eliminate thedisadvantageous effects of these two compounds when used individually ina plating solution and, surprisingly, without any disadvantageousoffsetting eifects as used with the brightening metals and with atetrachromate solution, and to operate effectively at relatively cold orlow solution temperatures (below about 32 C.). I enable operation athigher temperatures without spoiling the effectiveness of the solutionor its chemical complex. In this connection, the tetrachromate solutionis made up by adding sodium hydroxide or carbonate to a chromic acidaqueous solution, to thereby convert a greater part of the chromic acidto sodium tetrachromate, leaving free chromic acid in the range of aboutto below 50%.

As shown by examples, metals of the class of titanium and zirconium maybe used, as an optimum, in proportionate amounts together, and may alsobe used individually. Such metals may be introduced in the form offluorides, potassium titanate (K TiO potassium zirconate (K ZrO etc. Thefluoride portion of the complex may, as indicated, be provided by sodiumor silicon fluorides (to also provide silicon cation), or by fluoridesof the metals of the class of titanium and zirconium. The

selenium, if used, may be introduced as selenium dioxide. The followingare further examples of elfective plating solutions employing theprinciples of my invention:

Example 111 Total of chromic acid (CrO grams 150-600 Free chromic acid(H CrO with reference to the total chromic acid percent 20-50 Sulphatewith reference to the total chromic acid percent .07-.4 Fluoride grams.1-13 Titanium do .03-3 Zirconium do .04-6

Solution or bath operating temperature of about 18 to 39 C.

EXampleIV Same as Example HI, but also containing:

Silicon per liter of the electrolyte solution grams .02-2

Solution operating temperatures of about 30 to 39 C.

Example VI Same as Example V, but additionally containing:

Silicon per liter of electrolyte solution grams .02-2

The following are examples of chemical compounds that may be used inproviding a solution in accordanm with my invention:

Example .A

Grams Total chromic acid (CrO 200-270 Free chromic acid (H CrO 40-135Sulfuric acid (H .2-.8 Titanium fluoride (TiF .1-3 Zirconium fluoride(ZrF .1-8 Sodium fluoride (NaF) .1-8

Example B Grams Total chromic acid 420-600 Free chromic acid -210Sulfuric acid I .4-l.8 Potassium titanate (K TiO .1-10 Potassiumzirconate (K Zr-O .1-13 Silicon fluoride (SiF .l-10

Example C Grams Total chromic acid 280-300 Free chromic acid 60-140Sulfuric acid 1.5-2 Titanium fluoride .1-.5 Zirconium fluoride .l-1.2Selenium dioxide (SeO .007-.02

Example D Grams Total chromic acid 400-600 Free chromic acid 120-220Sulfuric acid 1.8-2.8 Potassium titanate .1-1 Potassium zirconate .1-2

Selenium dioxide .007-.02 Silicon fluoride .1-10

It will be noted that quantities or amounts of compounds containingmetals of the class of titanium and zirconium are reduced when aselenium compound is used in the electrolyte solution or bath. ExamplesI to VI and A to D all relate to tetrachromate solutions in which theproportioned or indicated ranges of total chromic acid (anhydride) andfree chromic acid (H CrO are obtained by the employment of a sodiumcompound of a type such as previously mentioned. All, in specifyinggrams, have reference to grams per liter of the aqueous solution orelectrolyte.

Since my procedure provides a first or initial chronium deposit on ametal surface that is crack-free in view of the low residue stress inthe deposit, it will retain this freedom from cracking and thus, providegreater corrosion protection. This is also true when the coated workpiece or article is subsequently subjected to a second chromium depositin further accordance with my invention. Such a second deposit is alsobright but has a micro-cracked surface pattern that will predictablyhave at least 1,000 micro-cracks per linear centimeter of surface, asviewed withabout a 50 power lens. In this manner, increased corrosionresistance is assured in view of the multiplicity of pore sites wherecorrosion may occur, thereby limiting corrosion current at individualcorrosion sites and thus minimizing the effectiveness of such action. Ifdesired, a solution such as represented by Example Il may be used toapply a first coating that has a minimum of 1,000 micro-cracks perlinear centimeter of surface.

When employing selenium in the solution, I have found that a slightlyhigher operating range is desirable, of about 30 to 32 C., as comparedto about 18 to 32 C. where it is not used. As indicated by the examples,smaller amounts of metals of the class of titanium and zirconium, atleast as to their upper limits, are used when selenium is employed.Although the titanium and zirconium have been designated as brighteners,they appear to have an influence on throwing power, control of stress,etc., as used with catalysts. In fact, the upper limits of these metalsare governed by their effectiveness in this connection and particularly,with reference to throwing power. A good, inexpensive, and workablesolution in accordance with my invention employs soluble titanium andsilicon compounds, with or without selenium. Selenium, when used, hasthe ability to partially displace metals of the class of titanium andzirconium; in a smaller quantity it serves as a brightener and in alarger quantity it serves as a brightener and a producer ofmicro-cracks. Silicon also has a brightening effect as used I in thesolution. My solution operates well at a much lower current density thanan ordinary tetrachromate basis solution. In this connection, it may beoperated from about 5 to about 50 to 60 amperes per square decimeter,

with about to 20 amperes as a good normal operating density. An optimumsolution temperature is below about 32 C. Also the good qualities of mydeposit including its high adherency are not limited to a maximum depth,and it may be applied to any suitable clean work piece surface, such asnickel, copper, brass, steel, cobalt, aluminum, etc. The work piece willbe the cathode, the anode may be of tin-lead or antimonial lead, and anysuitable voltage of, for example, 3 to 6 volts may be used in theplating operation.

Ordinarily the solution of my invention may be prepared by first addingchromic acid (H CrO to water, then adding the sodium compound topartially neutralize the chromic acid and form the tetrachromate whichis in the form of sodium tetrachromate but which is measured by testingfor the free chromic acid (H CrO content and calculated as anhydrideacid constant (CrO The brightening metals, such as titanium andzirconium, as well as the silicon and selenium additions may be made bydissolving soluble comp unds in water and then adding to the solution,or by dissolving them directly in the solution. The sulphate content maybe provided by adding sulfuric acid or sodium sulphate to the partiallyneutralized tetrachromate solution before the addition of the complexes.The fluoride may be added with the complexes in the form of solublecompounds. The selenium can be either directly dissolved in the chromicacid solution or separately dissolved in hot water and then added to thesolution.

Essentially, the solution of my invention enables the electro-chemicaldeposition of chromium metal on the surface that is always bright asdeposited, is corrosion and strain resistant, is highly adherent, iseffected with increased throwing power, and is crack and pore-free, butthat may be provided with micro-cracks, if desired. The coating isstress-free as applied. My solution enables avoidance of heretoforeundesirable limitations as to temperature of the solution, depth ofapplication, current density, etc., in that the chemical complexes, asused with the catalysts, in a tetrachromate solution in combination,produce entirely new and improved results including an asdepositedbrightness.

In carrying out my invention, I employ a complex of soluble compounds ina proportionated basis in a tetrachromate solution and that may haveabout 0.16 to 6 grams per liter of sodium fluoride, such as to provide acontent of free chromic acid to total chromic acid of a range of about 1to 3 up to about 1 to 5. My proportioned solution has an unusually goodcovering and bright throwing power at a high current efficiency of arange of about 18 to 22% of the theoretical, and deposits an adherentpore-free, single or double deposit (crack-free or microcracked), anddoes so effectively at lower temperatures. It is completely stable up toa maximum higher temperature of about 39 C. and has good stability up toa temperature of about C.

What I claim is:

1. A process for electrolytically depositing a chromium coating on ametal surface of a work piece which comprises, preparing an aqueoussodium tetrachromate electrolytic plating solution having a free chromicacid to total chromic acid content of about 1 to 3 up to about 1 to 5and which also contains: cations selected from the group consisting oftitanium and zirconium in sufiicient amount to produce a brightchrominum plate and sulfate and fluoride ions in .an amount sufiicientto increase the throwing power of the electrolyte, dissolving silicon inthe plating solution, introducing the work piece into the platingsolution as a cathode, applying electroplating current thereto anddirectly depositing a chromium coating on the surface of the work piecewhich is bright, resistant to corrosion, resistant to cracking, pore andstress free.

2. A process as defined in claim 1 wherein silicon is introduced insoluble form into the plating solution in a minor .amount of about .02to 2 grams/ liter.

3. A process as claimed in claim 1 wherein the workpiece is thereaftertreated as cathode in a second plating solution, said plating solutionhaving a sufiicient but small amount of a soluble selenium compound toform microcracks in .a second deposited coating, and applyingelectroplating current thereto until a micro-cracked chromium coating isdeposited.

4. A process as defined in claim 1 wherein the tetrachromate of thesolution is provided by reacting chromic acid with a compound of theclass consisting of sodium hydroxide and sodium carbonates.

5. A process as defined in claim 1 wherein selenium is dissolved in thesolution.

6. A process as defined in claim 1 wherein the plating solution ismaintained below a temperature of about 32 C.

7. A process for electrolytically depositing a chromium. coating on ametal surface of a work piece which comprises, preparing an aqueoussodium tetrachromate electrolytic plating solution of increased throwingpower hav ing a free chromic acid to total chromic acid content of about1 to 3 up to about 1 to 5 Which also contains:

cations selected from the group consisting of titanium and zirconium inan amount of about .03 up to about 3 grams/ liter when the cation istitanium and in an amount of about .04 up to about 6 grams/liter whenthe cation is zirconium, about .07 up to about .7% sulfate ions withrespect to the total chromic acid anhydride, and about .07 up to about13 grams/liter of fluoride ions; introducing the work piece into theplating solution as a cathode, applying electroplating current thereto.and directly depositing a chromium coating on the surface of the workpiece which is characterized by its brightness, improved resistance tocorrosion, freedom from pores, resistance to cracking and freedom frominternal stress and strain.

8. A process as defined in claim 7 wherein a small minor amount ofselenium is introduced in soluble form into the plating solution tofurther improve the deposited coating.

9. A process as defined in claim 7 wherein about .02 to 2 grams ofsilicon per liter of electrolyte are provided in the plating solution.

10. A process as defined in claim 7 wherein, the plating solution isprovided with a content of metals of the class consisting of titaniumand zirconium in an amount of about .03 to .3 gram per liter for thetitanium and about .04 to .9 gram per liter for the zirconium, about.005 to .015 gram per liter of selenium per liter of the platingsolution, and the plating solution is maintained at a temperature ofabout 30 to 39 C. during the deposition of the chromium coating.

11. A process as defined in claim 10 wherein the plating solution isadditionally provided with about .02 to 2 grams of silicon in solubleform per liter of plating solution.

12. A process as claimed in claim 7 wherein the workpiece is thereaftertreated as cathode in a second plating solution, said plating solutionhaving .a sufiicient but small amount of a soluble selenium compound toform microcracks in a second deposited coating, and applyingelectroplating current thereto until a micro-cracked chromium coating isdeposited.

13. A process as claimed in claim 7 wherein the workpiece is thereaftertreated in a second plating solution, said second plating solution beingan aqueous tetrachromate electrolytic plating solution having a freechromic acid to total chromic acid content of about 1 to 3 up to about 1to which also contains: cations selected from the group consisting oftitanium and zirconium in an amount of about .03 up to about .15gram/liter when the cation is titanium and in an amount of about .13 upto about .53 gram/liter when the cation is zirconium, about 1.66 up toabout 1.86 grams/liter of sulfate ions, about .07 up to about .67gram/liter of fluoride ions, and about .01 up to about .02 gram/liter ofa selenium compound; and introducing the work piece into the secondplating solution as a cathode, applying electroplating current thereto,and directly depositing a micro-cracked chromium coating.

14. In a process as defined in claim 7, providing the plating solutionwith a content of about 150 to 600 grams/ liter total amount of chromicacid anhydride and about 20 to 50% of free chromic acid with respect tothe total of chromic acid anhydride, and maintaining the temperature ofthe plating solution at about 18 to 32 C. during the depositing of thechromium coating.

15. A process for electrolytically depositing a chromium coating on ametal surface of a work piece wherein the coating is characterized byits as-plated brightness, its high adherency, by its resistance tocracking, and by its freedom from pores which comprises, preparing .anaqueous tetrachromate plating solution containing about 200 to 270 gramsper liter of total chromic acid anhydride, about 70 to 90 grams perliter of free chromic acid, about .6 to .8 gram per liter of sulfuricacid, a metal compound of the class consisting of titanium and zirconiumfluorides in an amount of about 1.5 to 3 grams per liter of titaniumfluoride and about 4 to 8 grams per liter of zirconium fluoride, andselenium in soluble form in an amount of about .003 to .005 gram perliter; maintaining the plating solution at a temperature below about 32C., introducing the work piece into the plating solution and employingit as a cathode while applying electric current thereto, directlydepositing a bright chromium coating on the work piece in the solution,and removing the work piece from the solution.

16. A process for electrolytically depositing a microcracked coating on.a metal surface of a work piece which comprises, preparing an aqueoustetrachrom ate plating solution containing about 280 to 320 grams perliter of chromic acid anhydride, about to grams per liter of freechromic acid, about 1.7 to 1.9 grams per liter of sulfuric acid, a metalcompound of the class consisting of titanium and zirconium fluorides inan amount of about .1 to .5 gram per liter of titanium fluoride andabout .3 to 1.2 grams per liter of zirconium fluoride, and about .01 to.02 gram per liter of selenium in a soluble form; maintaining theplating solution at a temperature below about 39 C., introducing thework piece into the plating solution and employing it as a cathode whileapplying electric current thereto, directly depositing a bright chromiumcoating on the work piece in the solution, and removing the Work piecefrom the solution.

17. A process for electrolytically depositing a microcracked chromiumcoating on a compatible metal surface of a work piece which comprises,preparing an aqueous sodium tetraohromate electrolytic plating solutionhaving a free chromic acid to total chromic acid content of about 1 to 3up to about 1 to 5 which also contains: cations selected from the groupconsisting of titanium and zirconium in an amount of about .03 up toabout .15 gram/ liter when the cation is titanium and in an amount ofabout .13 up to about .53 gram/liter when the cation is zirconium, about1.66 up to about 1.86 grams/liter of sulfate ions, about .07 up to about.67 gram/ liter of fluoride ions, and about .01 up to about .02gram/liter of a selenium compound; and employing the work piece as acathode within the plating solution while applying electric currentthereto and directly depositing a micro-cracked chromium coating on thecompatible metal surface of the work piece.

References Cited UNITED STATES PATENTS 1,985,308 12/1934 Bornhauser204-51 2,686,756 8/1954 Stareck et al 20451 2,800,443 7/1957 Stareck eta1. 204-51 2,9,52,590 9/1960 Stareck et a1 204-51 2,962,428 11/ 1960Passal 2045l 3,157,585 11/1964 Durham 204-41 FOREIGN PATENTS 798,590 7/1958 Great Britain. 1,106,575 5/1961 Germany.

654,782 12/1962 Canada.

OTHER REFERENCES Safranek, W. H., et al.: Use of Selenic Acid forPlating Microcracked, Protective and Decorative Chromium Plate, Plating,pp. 1027-1031, September 1960.

HOWARD S. WILLIAMS, Primary Examiner. G. KAPLAN, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,418,220 December 24, 1968 Wilhelm Roggendorf It is certified thaterror appears in the above identified patent and that said LettersPatent are hereby corrected as shown below:

Column 4, line 31, after 1.5-3" insert or line 51, after "0.1-0.5"insert or Column 6, line 11, after ".03-3" insert or line 28 after 03-.3" insert or line 46, after ".l-3" insert or line 55, after ".1- l0"insert or line 64, after ".l-.5" insert or line 72, after ".l-l" insertor Column 7, line 12, "chronium" should read chromium line 71,"constant" should read content Signed and sealed this 10th day of March1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E.

Attesting Officer Commissioner of Patents

